The National Park Service and its supporting charity, the National Park Foundation, are once again throwing the gates open to the United States' national parks. The weekend of April 19 and 20 will see fees waived across the country.
Some particularly timely national park units to visit are the ones that are marking the 150th anniversary of Civil War battles this year, including Petersburg in Virginia.
Of the USA's 401 national park units, 133 charge entrance fees. Yosemite, to take a popular example, usually costs $20 per car.